1. Field of the Invention
The present invention relates generally to a communication system using an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and in particular, to an apparatus and method for transmitting/receiving channel quality information (CQI) for subcarriers between a mobile subscriber station (MSS) and a base station (BS).
2. Description of the Related Art
With the rapid progress of mobile communication systems, the required amount of data and its processing speed are increasing rapidly. Generally, when data is transmitted over a wireless channel at a high speed, the data experiences a high bit error rate (BER) due to multipath fading and Doppler spread. A wireless access scheme appropriate for the wireless channel is required to compensate for the high BER, so a Spread Spectrum (SS) scheme having advantages of lower transmission power and lower detection probability is becoming popular.
The SS scheme is roughly classified into a Direct Sequence Spread Spectrum (DSSS) scheme and a Frequency Hopping Spread Spectrum (FHSS) scheme.
The DSSS scheme can actively adjust to a multipath phenomenon occurring in a wireless channel using a rake receiver that uses path diversity of the wireless channel. The DSSS scheme can be efficiently used at a transfer rate of 10 Mbps or less. However, when transmitting data at a rate of 10 Mbps or higher, the DSSS scheme increases in inter-chip interference, causing an abrupt increase in hardware complexity. Also, it is known that the DSSS scheme has a limitation in user capacity due to multiuser interference.
The FHSS scheme can reduce multichannel interference and narrowband impulse noise because it transmits data, hopping between frequencies with random sequences. In the FHSS scheme, correct coherence between a transmitter and a receiver is very important, but it is difficult to achieve coherent detection during high-speed data transmission.
An Orthogonal Frequency Division Multiplexing (OFDM) scheme is a scheme appropriate for high-speed data transmission in a wire/wireless channel, and on which extensive research is being conducted. The OFDM scheme transmits data using multiple carriers, and is a type of a Multi-Carrier Modulation (MCM) scheme that parallel-converts a serial input symbol stream into parallel symbols and modulates the parallel symbols with a plurality of narrower-band subcarriers having mutual orthogonality before transmission. A subcarrier in a specific time interval is referred to as a “tone.”
The OFDM scheme has high frequency efficiency because it uses a plurality of subcarriers having mutual orthogonality as described above. Because a process of modulating/demodulating the plurality of subcarrier signals is equivalent to a process of performing an Inverse Discrete Fourier Transform (IDFT) and a Discrete Fourier Transform (DFT), a transmitter and a receiver can modulate and demodulate the subcarrier signals at a high speed using the Inverse Fast Fourier Transform (IFFT) and the Fast Fourier Transform (FFT), respectively.
Because the OFDM scheme is appropriate for high-speed data transmission, it has been adopted as a standard scheme under the Institute of Electrical and Electronics Engineers (IEEE) 802.11a standard, the HIPELAN/2 High-Speed Wireless Local Area Network (LAN) standard, the IEEE 802.16 standard, the Digital Audio Broadcasting (DAB) standard, the Digital Terrestrial Television Broadcasting (DTTB) standard, the Asymmetric Digital Subscriber Line (ADSL) standard, and the Very-high data rate Digital Subscriber Line (VDSL) standard.
In a communication system using the OFDM scheme (hereinafter referred to as an “OFDM communication system”), a structure of a frequency domain of an OFDM symbol utilizes subcarriers. The subcarriers are divided into data subcarriers used for data transmission, pilot subcarriers used for transmitting symbols in a predefined pattern for various estimation purposes, and null subcarriers for a guard interval and a static component. All of the subcarriers except for the null subcarriers, i.e., the data subcarriers and the pilot subcarriers, are effective subcarriers.
An Orthogonal Frequency Division Multiple Access (OFDMA) scheme, which is a multiple access scheme based on the OFDM scheme, divides the effective subcarriers into a plurality of subcarrier sets, i.e., subchannels. The “subchannel” refers to a channel comprised of at least one subcarrier, and the subcarriers constituting the subchannel may be either adjacent to each other, or not adjacent to each other. A communication system using the OFDMA scheme (hereinafter, referred to as an “OFDMA communication system”) can simultaneously provide services to a plurality of users.
A general subchannel allocation structure in the OFDMA communication system will now be described with reference to FIG. 1.
Referring to FIG. 1, the subcarriers used in the OFDMA communication system include a DC subcarrier representing a static component in a time domain, the subcarriers representing a high-frequency band of a frequency domain, i.e., a guard interval in the time domain, and the effective subcarriers. The effective subcarriers constitute a plurality of subchannels, and in FIG. 1, the effective subcarriers constitute three subchannels, i.e., a subchannel #1 to a subchannel #3.
The OFDMA communication system uses an Adaptive Modulation and Coding (AMC) scheme in order to support high-speed data transmission through a wireless channel. The AMC scheme refers to a data transmission scheme for adaptively selecting a modulation scheme and a coding scheme according to a channel state between a cell, i.e., a base station (BS), and a mobile subscriber station (MSS), thereby increasing the entire cell efficiency.
The AMC scheme has a plurality of modulation schemes and a plurality of coding schemes, and modulates and codes channel signals with an appropriate combination of the modulation schemes and the coding schemes. Commonly, each combination of the modulation schemes and the coding schemes is referred to as a Modulation and Coding Scheme (MCS), and a plurality of MCSs with a level 1 to a level N are defined by the number of the MCSs. One of the MCS levels is adaptively selected according to a channel state between a BS and an MSS wirelessly connected to the BS.
In order to use the AMC scheme, an MSS should report a channel state, i.e., CQI (Channel Quality Information), of a downlink to a BS. In the current IEEE 802.16 communication system, it is provided that an MSS should report the CQI of a downlink to a corresponding BS using a Report Request/Report Response (REP-REQ/REP-RSP) scheme.
That is, a BS transmits an REP-REQ message to a particular MSS, and the MSS transmits an REP-RSP message including the CQI of a downlink to the BS in response to the REP-REQ message. For example, the CQI can include an average value and a standard deviation value of a carrier-to-interference and noise ratio (CINR) or a received signal strength indicator (RSSI).